Drying oven with catalytic exhaust treatment



April 1964 w. c. VERNER ETAL 3,130,961

DRYING OVEN WITH CATALYTIC EXHAUST TREATMENT Filed Jan. 16, 1 962 2 Sheets-Sheet 1 Figure r .wag

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William 0. Verne! James L. Donahue 39 [39 a BY Walter A. Del/my I 38 i y w 38 A ruin/v? Adjustable 5/0! Size A ril 28, 1964 w. c. VERNER ETAL DRYING OVEN WITH CATALYTIC EXHAUST TREATMENT 2 Sheets-Sheet 2 Filed Jan. 16, 1962 Z w I, R E l .N 3 T 0 N0 M x N E V m 5 r Mm ,A 7 .lmfl v 1 I l v ,x B

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m sag United States Patent William C. Vernier, Detroit, James H... Donahue, Warren, ter A. -t. tjiair Shores, Mich assignors Products Lornpany, Des Plaines, llil., of Eelaware i6, Ser. N 166,534

6 Claims. (Cl. 263-6) This invention relates to apparatus for heating coated articles or particles of material to evaporate and oxidize volatile components or coatings from such articles or material. More specifically, the improved apparatus provides a compact single housing and single burner arrangement that channels the hot gas streams in a manner permitting maximum use of heat input to the unit while effecting the oxidation of combustible volatile material recovered from the heat treating section.

The present apparatus is particularly adapted to effect the removal of cutting oils and machine oils that adhere to metal turnings and chips from machine shop operations. Machine oils and cutting oils clinging to metal turnings must be removed before such scrap metal can be emelted and reused. In order to preclude shipping or transporting such scrap material any great distance, it is frequently advantageous to maintain so called chip drying apparatus in machine shop installations to effect the removal of such oils and thus permit the immediate reuse of such metal. Many of the present types of dryers for metal chips and turnings are of a rotary type, having means for accommodating the material in a rotating drum arrangement that is subjected to external heat. Thus, the cutting oils are vaporized and driven oif the metal material permitting the latter to be removed and subsequently subjected to heating and melting while substantially free of contaminating oily films. Such rotating drum types of Vaporizers and dryers are generally ineflicient in operation in that there is no burning of the combustible components that are vaporized from the scrap material, nor is there purification of the exhaust gas stream.

The construction and arrangement of the present improved drying apparatus is also advantageously made use of in effecting the drying of varnish or lacquer type coatings which are applied to metal sheets or strips for the manufacture of cans or other metal containers. For example, in the manufacture of beer cans, coffee cans, oil cans, fruit juice containers, and the like, there is a protective lacquer coating applied to the sheet material before the formation of the container. Also in applying decorative coatings, there may be volatile materials that must be dried from such coatings applied to the sheet material. Decorative applications may be in addition to one or more layers of primer coatings or protective coatings.

It is a principal object of the present invention to provide an improved form of evaporating and drying apparatus which incorporates a catalytic element therein to assist in the efficient substantially low temperature catalytic incineration of inflammable and obnoxious fumes present in the system.

It is also an object of the present invention to provide a construction and arrangement in an evaporating and oxidizing apparatus which utilizes a single compact insulated housing with a single burner position, such that the apparatus is adapted to dry and incinerate volatile material from material that is maintained in continuous movement on a conveyor system.

It is a still further object of the present invention to utilize blowers and duct work arrangements within a single housing unit so as to effect a countercurrent heat exchange flow between a hot flue gas stream and an exhaust gas "ice stream, as well as obtain recirculation of the used hot gas stream in a manner having such stream, together with entrained combustible vapors, discharged at the zone of the burner means to in turn obtain rapid reheating thereof and the combustion of a portion of the inflammable material recirculated therewith.

Broadly, the present invention embodies a drying and gas purifying apparatus which comprises in combination, a substantially enclosed and confined housing, with said housing having an open-ended horizontal passageway extending through the lower portion thereof, such passageway being adapted to accommodate a moving stream of material that is to be heat treated as it passes through the housing, conveyor means extending through said passageway for carrying the material to be heated, burner means positioned in and extending through one end of the housing and directing a hot line gas stream into the interior thereof, a pair of spaced blowers mounted within the housing at an end thereof opposing the burner means, hot gas inlet means to each of the blowers, a hot gas duct means from the discharge of one of the blowers extending downwardly to a hot gas distributing chamber, said distributing chamber positioned above and co-extensive with said horizontal passageway and having a plurality of outlet louvers along its lower face, a gas collecting chamber positioned below the horizontal passageway and co-extensive therewith having a plurality of spaced louvers along the upper face thereof, a return gas duct extending along the interior side portion of the housing from said gas collecting chamber to a gas outlet at the interior end portion of the housing at the zone adjacent said burner means whereby a recirculated gas stream mixes with the hot line gas stream from said burner means, an exhaust gas duct positioned within the housing and extending from the outlet of the other blower of said pair to an oxidized gas outlet section, a permeable layer of oxidation catalyst positioned across the end portion of the exhaust gas duct and providing thereby catalytic treatment of the entire gas stream passing through said exhaust gas duct, and a treated gas outlet from the oxidized gas outlet section and said housing.

In more specific embodiments, additional advantageous means may be incorporated in the improved apparatus arrangement to effect adjustability in tempering the internal temperature as well as an efiicient operation in obtaining substantially complete oxidation of combustible oil vapors which may be obtained from the heat treating section of the apparatus. As for example, an outlet port with an adjustable damper means may be provided in the hot gas recirculation gas duct such that a portion of the gas stream may be recirculated to the blower end of the housing and entirely by-pass the burner zone. Means may also be provided for introducing fresh air into the system at the blower end of the housing to supply cooling air that will mix with and dilute the hot gas treating stream. A preferred apparatus construction also provides adjustment means at the zone of the louvers from the hot gas distributing chamber so as to control the quantity and flow pattern of the hot gas flow contacting the material being heat treated in the apparatus.

The construction and arrangement of one embodiment of the improved drying and catalytic treating apparatus may be better described, and the advantages thereof set forth, by reference to the accompanying drawing and the following description thereof.

FIGURE 1 of the drawing is a diagrammatic plan view with the top removed, indicating the general arrangement of ducts, burner, and blowers, etc., within the single housing arrangement.

FIGURE 2 of the drawing is a sectional elevational view through hte housing, as indicated by the line 2-2 in FIGURE 1.

FIGURE 3 is a cross-sectional elevation view through one end of the apparatus, as indicated by the line 33 in FIGURE 1.

FIGURE 4 is also a cross-sectional elevational view through the housing, as indicated by line 4-4 in FIG- URE 1.

FIGURE is a partial sectional View through the exhaust gas duct and treated gas outlet section as, indicated by line 55 in FIGURE 1.

FZGURE 6 shows a design and construction used in providing an adjustable gas outlet louver.

:FIGURE 7 illustrates a dual belt or tray arrangement for the conveyor means or" the unit.

Referring now to the drawings, there is shown a substantially rectangularly shaped housing 1 which is indicated as having a thick layer of insulating material .2 in :order that there is a maximum amount of heat retained Within the interior portion of the housing. As is best shown in the elevational sections, an open-ended heat treating section or passageway 3 is provided longitudinally through the lower portion of housing 1 and adapted to accommodate a movable conveyor belt 4. The conveyor belt or grid 4, as used in combination with the present apparatus, is preferably formed of a permeable screen-like belting or grating that permits the passage of a gaseous stream therethrough, while at the same time supporting a layer of material to be heat treated. The present drawings indicate diagrammatically that a layer of material it} such as metal chips and turnings, is fed to the top of grating 4- from another conveyor means 5 so that the material is carried horizontally through the passageway 3 and out the opposite end of the housing 1. It is not intended, however, to limit the present improved heat treating apparatus to use with any one type of conveyor means or to use with anw one type of preforate grid or grating, inasmuch as the latter is of necessity constructed and designed to hold a particular type and size of material to be placed thereon for movement and treatment through the passageway 3. Where individual metal sheets, stain-pings, or unitary pieces are to be heated, then suitable conveyor means are used that can hold and suspend the individual units in a spaced relationship as they are transported in passageway 3 for the heat treating operation. A suitable opening 6 is provided at one end of the housing 1 and in the insulation 2 to accommodate the introduction of the material 10 on the conveyor means 4, and similarly an outlet opening 7 is provided at the opposite end of the housing 1 to accommodate the discharge of the treated material.

At an end of the housing 1, there is provided burner means 8 in combination with a suitable burner block 9, that in turn provides for the introduction of a hot flue gas stream into the interior of housing 1 and a source of hot gases for contacting the material It) in the heat treating zone 3. Natural gas or other suitable fuel is supplied to burner 8 by supply line 11 and control valve 12. A portion of the hot flue gas stream from burner means 8 passes to an opposing hot gas fan or blower 13 which is positioned with the interior housing 1 for the purpose of circulating continuously and substantially uniformly a hot gas stream to the heat treating section or material passageway 3. A partition 41 is positioned vertically in the blower end of housing 1, between blowers 13 and 24, so as to separate the inlet stream to each of the respective blowers. The partition 4-1 also permits tempering and dilution of the hot gas stream to the treating zone, as will be set forth in more detail hereinafter.

The blower 13 may be of a conventional multiple blade centrifugal type which is adapted to accommodate high temperature gases, having an intake at the side thereof and a discharge end which connects with a plenum chamber 14. The latter extends laterally and downwardly, connecting with a longitudinal gas distributing chamber 15 which is positioned above and CO-XiIlSlVe with the i heat treating passageway 3. Chamber i5 is constructed and arranged to have a width which is about equal to or slightly greater than grid means 4 such that the hot gas stream may be unformly distributed through a perforated or louver plate 16 along the bottom portion thereof. The distributed gas stream then passes through the material ll) superimposed on grid 4 and into a gas reeiving chamber 17. The latter has a perforated or louvered plate 18 along its upper face and is co-extensive with the passageway 3 so that there is a substantially uniform intake of the heating stream together with vaporized volatile material recovered from the treated material 10. The lower longitudinal gas collecting duct 17 is provided with an outlet 19 which communicates with a vertical gas recirculation duct 2) that in turn conieets with a recirculation gas passageway 21 formed between pantition plate 22 and the side wall of the housing 1.

As is best shown in FIGURE 2 of the drawings, the partition plate 22 terminates short of the end of housing 1 so as to provide an opening .23 which is adjacent the burner block 9 and burner means 8 whereby the recirculated gas stream is mixed directly with the hot line gases from burner 8. This arrangement effects the reheating of the recycled stream and in addition permits a portion of the inflammable combustible materials to be directly oxidized by the burner means and provide additional high temperature gases within the main portion of housing 1. The susbtantially complete oxidation of volatile material is effected catalyticaliy in the exhaust section, thus precluding the discharge of combustible and obnoxious tunes to atmosphere.

The portion of the hot gas stream passing to the blower end of the housing 1 i.e., that portion which is on the side of the partition 41 away from the hot gas fan 13 and distributing duct 15 is passed through the second blower of the pair, indicated as the exhaust fan 24, and is then discharged into exhaust passageway 25. The latter is formed within the interior side portion of the housing 1 by vertical partition plate as, lower plate member 27. The exhaust passageway 25, being entirely within insulated housing 1, is also in a heat exchange relationship and in a reverse flow arrangement with the hot gases passing from burner 8. Positioned vertically across the downstream end of exhaust duct 25 is a permeable layer of oxidation catalyst indicated as element 28. The catalytic element is preferably constructed and arranged so that it is removable from the housing for purposes of cleaning or replacement, being indicated in the present embodiment as supported between the continuous set of angle members 2& positioned around the periphery of exhaust duct 25. Downstream from the catalyst element 28 is a gas outlet section 3% which communicates with a gas outlet stack 31, superimposed over the top of housing 1, whereby resulting catalytically oxidized fumes are discharged from the housing substantially free of combustible material.

It is not intended to limit the present invention to the use of any one type of catalytic material or any one form of catalyst element. Granular or pellet types of heat resistant catalytic materials may be placed in a suitable pervious container that extends across the passageway and provide the desired substantially complete catalytic oxidation of combustible material entrained in the exhaust gas stream. A preferred form of catalytic element may comprise the type set forth in the Suter et al. Patent No. 2,658,742 dated November 10, 1953. Such patent discloses the use of a catalytically activated metallic alloy form of catalyst element, and it is not believed necessary to describe in detail the formation and construction of this type of catalytic incinerator unit, reference being made to the patent itself for this purpose.

For purposes of providing a tempering of the hot gas flow stream to the heat treating section 3, provision is made for introducing air into the interior of housing 1 by means of a fresh air inlet duct 32, having adjustable damper 33 that is in turn manually controlled by damper handle 34. The fresh air inlet duct 32 is indicated as being positioned at the blower end of housing 1, on the side of the partition 41 away from exhaust fan 24, such that the fresh air is available to mix with the hot gas stream entering blower 13. The present design with heat exchange Wall 26 and with partition 41 preventing tempering of the exhaust gas stream to blower 24, assures a high temperattu'e stream to the catalyst element 28 and a sustained oxidation of combustible fumes.

A preferred embodiment of the apparatus, as previously indicated, also provides means for shunting a portion of the recirculating gas stream directly to the inlet end of blower 13, thus by-passing the gas heating burner 8. At the upper end portion of passageway 21 there is provided an outlet port 35 with an adjustable damper 36. The latter is indicated as being manually adjustable by handle 37 extending through the side wall of housing 1 and the insulation 2, whereby there is external control of the amount of the recycled hot gas stream going to the blower end of the housing, rather than to the burner end thereof. Thus, damper means 36 provides a further means, in addition to fresh air inlet damper 33, for adjusting or tempering the temperature of the hot gas flow stream passing to blower 13 and the heat treating passageway 3.

In FlGURE 6 of the drawing there is indicated one means for providing adjustability in the outlet louvers, i.e., outlet area in the lower face of the hot gas distribution duct 15. A plurality of spaced lower plates 38 are arranged to support superimposed plates 3? in engagement with one another. There is thus provided adjustable spacing of the edge of a plate 39 with an opposing adjacent longitudinal edge of a plate 38 and in turn provide desired sized slotted openings through the bottom face of the duct 15. After a desired outlet arrangement and flow rate has been established from the gas distribution chamber 15 and through the materifl 1i passing on the moving grid 4, then plates 38 and 39 can be bolted or otherwise fastened together to maintain the desired spacing.

In connection with an apparatus which is utilized to dry metal turnings, chips, etc., another advantageous modification of the unit is set forth, and illustrated in part, by reference to FIGURE 7 of the drawing. In FIGURE 7 there is shown diagrammatically, in a partial longitudinal section, a movable dual conveyor tray-belt arrangement having perforate upper sections 42 and non-perforate lower sections 43. The upper sections are formed of perforate metal adapted to hold the material 1%) in a manner such that the hot gas treating stream from the gas distributing chamber will pass through the material substantially uniformly. The lower sections 4-3 of the present embodiment are spaced below the upper sections and are of a non-perforate construction, as shown, such that any finely divided material ll), from the layer 1% that may fall through the perforate portions of sections 42 can be separately collected and removed from the discharge end of the conveyor means. The upper sections 42 are indicated as being tack-welded along their sides to the upturning side wall portions 44 of lower sections 43 to provide a double tray construction. Also, the leading edge (with reference to the direction of the conveyor movement) of each of the sections 42 and 43 are bent slightly upward and are spaced apart such that transverse slots or openings 45 permit chip tines to be dumped at the unloading end of the unit, however, it is not intended to limit the improved dual conveyor arrangement to only one design inasmuch as variations in actual fabrication are obvious. For example, various combinations of perforate belting or grating may be used in a spaced dual arrangement rather than using the two separate traylike sections. With perforations in both the upper and lower belt sections of the conveyor means, there is permitted a more direct flow of hot gases therethrough to the gas receiving chamber and improved drying and/ or treating of the material It).

In many cases, in the use of an apparatus of the type of the present invention, it is desirable to remove fines, such as ill, which fall through the upper sections 4-2, by reason of such fines being ultimately harmful or nonusable, and thus preferably removed from the bulk of the material iii. For example, fine particles of aluminum chips tend to cause excessive foaming in a remelt furnace and a higher formation of dross. t may thus be seen that the dual section arrangement of FIGURE 7 is an advantageous modification for use with certain types of materials which are to be spread across the surface of the conveyor means for the heating operation.

Still another feature of the present improved apparatus is the use of one or more explosion doors in connection with housing 11. An explosion door as, which may be hingedly attached to the housing, is indicated as positioned on the top of housing ll above the hot gas flow path therein. It is not intended, however, to limit the present invention to the use of any one type of explosion door or to any one location. Further, it is not intended to limit the present construction and arrangement to the use of any one type of burner, or to the use of a single burner at the burner end of the housing. It is, however, a feature of the improved construction and arrangement to utilize burner means at one end of the chamber only, such that a hot flue gas source is positioned at one location in the housing, substantially opposing the blower end thereof, and in a position adjacent to the outlet from the gas recirculation duct carrying the volatilized components from the material being heat treated in the passageway 3.

The drawings do not show motors for the respective fans 13 and 24, however, suitable electrical motors may be mounted externally from the housing, with one on each side, thereof, to effect the operation of the two fans or blowers, l3 and 24, maintained internally within the housing. lso, where desired suitable electrical control means may be used in connection with such motors to provide means for starting and stopping the motors for the fans responsive to the movement of the conveyor system, or the stopping and starting of the power means which operates the conveyor belt 4.

We claim as our invention:

1. A drying and gas purifying apparatus, comprising in combination, a housing, an open horizontal passageway through the lower portion of said housing, having an entrance opening to said passageway and an outlet opening therefrom for passing material to be treated through said housing, a conveyor means having a belt portion extending through said passageway for the carrying of material to be treated, a burner positioned through one end of said housing and providing for directing a hot flue gas stream into the interior thereof, a pair of spaced blowers mounted within said housing at the end opposing said burner, gas inlet means to each of said blowers from the interior of said housing, duct means from the discharge of one of said blowers extending downwardly to a hot gas distributing chamber, said chamber positioned above and co-extensive with said horizontal open passageway and having a plurality of outlet openings spaced along its lower face, a gas collecting chamber having a plurality of inlet openings spaced along the upper face thereof and positioned directly below said passageway and said belt portion of said conveyor means, a return gas duct extending along the interior side portion of said housing from said gas collecting chamber to a gas outlet discharging at the interior end portion of said housing at a position adjacent said burner whereby a recirculated gas stream mixes with said hot flue gas stream from said burner, exhaust gas duct means positioned within said housing and extending from the outlet of the other blower of said pair to an oxidized gas out- 7 let section, a permeable layer of an oxidation catalyst positioned across the end portion of said exhaust gas duct at the inlet to said oxidized gas outlet section, and a treated gas outlet from the latter section and from said housing.

2. A drying and gas purifying apparaus, comprising in combination, an insulated housing, an open-ended horizontal passageway through the lower portion of said housing, having an entrance opening to said passageway and an outlet therefrom for passing material to be treated through said housing, a conveyor means having a gas permeable belt portion positioned to continuously move horizontally through said passageway of said housing and carry said material to be treated, a burner positioned through one end of said housing providing for the introduction of a hot flue gas stream into the interior thereof, a hot gas blower and a gas exhaust fan within the end of said housing opposing said burner, separate hot gas inlet means to both said blower and said exhaust fan from the interior of said housing, a duct from the discharge of said hot gas blower extending downwardly to a hot gas distributing chamber, said chamber positioned above and co-extensive with said horizontal passageway through said housing and having a plurality of hot gas outlet louvers spaced horizontally along its lower face, a gas collecting chamber positioned below and co-extensive with said horizontal passageway and said belt portion of said conveyor means, said gas collecting chamber having a plurality of spaced louvered openings in the upper face thereof, a return gas duct extending along the interior side portion of said housing from said gas collecting chamber to a gas outlet discharging at the interior end portion of said housing at a zone adjacent said burner whereby a recirculated gas stream mixes with the hot flue gas stream from said burner, an exhaust gas duct positioned within said housing and extending from the outlet of said exhaust gas fan to an oxidized gas outlet section, said exhaust gas duct passing in heat exchange relationship with the hot flue gas stream from said burner, a permeable catalytic fume oxidizing layer removably supported across said exhaust gas duct at the inlet to said oxidized gas section, and a treated gas outlet from the latter section and from said housing.

3. The apparatus of claim 2 further characterized in that a recirculated gas outlet port is provided in said return gas duct adjacent the gas blower end of said housing, and adjustable damper means is provided in said-outlet port providing for the adjustable flow control of a portion of the recirculated hot gas stream to said blower end of said housing.

4. The apparatus of claim 2 further characterized in that said hot gas outlet louvers from said gas distributing chamber have movable and adjustable plate members whereby to vary the gas outlet area therefrom.

5. The apparatus of claim 2 further characterized in that vertical partitioning means is positioned between said hot gas blower and said gas exhaust fan, a fresh air inlet means with adjustment means thereto is positioned through the blower end of said housing adjacent said hot gas blower and on the side of said vertical partitioning away from said exhaust fan.

6. A drying and gas purifying apparatus adapted to effect the heating of a particulate material while it is maintained in a continuously moving layer, which comprises in combination, an insulated housing, an open-ended horizontal passageway through the lower portion of said housing having an entrance opening to said passageway and an outlet opening therefrom for passing material to be treated through said housing, a burner positioned through one end of said housing providing for the introduction of a hot flue stream into the interior thereof, a hot gas blower and a gas exhaust fan positioned within the end of said housing opposing said burner, separate hot gas inlet means to said blower and to said exhaust fan from the interior of said housing, a duct extending from the discharge of said hot gas blower downwardly to a hot gas distributing chamber, said chamber being positioned above and coextensive with said horizontal passageway through said housing and having a plurality of adjustable gas outlet louvers spaced horizontally along the lower face thereof, a dual conveyor means having upper and lower belt sections positioned to continuously move horizontally through said open-ended passageway below said gas distributing chamber, said upper sections of the conveyor means having a perforate gas permeable construction and means for carrying the bulk of said particulate material in a layer thercacross and the lower belt sections of said conveyor means being non-perforate and spaced a short distance below said upper sections, a gas collecting chamber positioned below and co-extensive with said horizontal passageway and the portion of said dual conveyor means carrying material therethrough, said gas collecting chamber having a plurality of spaced louvered openings on the upper face thereof, a return gas duct extending along the interior side portion of said housing from said gas collection chamber to a recycle gas outlet discharging at the interior end portion of said housing at a zone adjacent said burner whereby a recirculated gas stream mixes with the hot flue gas stream from said burner, an exhaust gas duct positioned within said housing opposite said return gas duct and extending from the outlet of said exhaust fan to an oxidized gas outlet section, said exhaust gas duct passing in heat exchange relationship with the hot flue gas stream passing from said burner end of said housing, a permeable catalytic fume oxidizing layer removably supported across said gas duct at the inlet to said oxidized gas section, and a treated gas outlet from the latter section and from said housing.

References Cited in the file of this patent UNITED STATES PATENTS 2,005,082 Greenawalt June 18, 1935 2,558,338 Clements June 26, 1951 2,658,742 Suter et a1 Nov. 10, 1953 2,926,897 Barnett et al Mar. 1, 1960 2,950,098 Ruff Aug. 23, 1960 

1. A DRYING AND GAS PURIFYING APPARATUS, COMPRISING IN COMBINATION, A HOUSING, AN OPEN HORIZONTAL PASSAGEWAY THROUGH THE LOWR PORTION OF SAID HOUSING, HAVING AN ENTRANCE OPENING TO SAID PASSAGEWAY AND AN OUTLET OPENING THEREFROM FOR JKPASSING MATERIAL TO BE TREATED THROUGH SAID HOUSING, A CONVEYOR MEANS HAVING A BELT PORTION EXTENDING THROUGH SAID PASSAGEWAY FOR THE CARRYING OF MATERIAL TO BE TREATED, A BURNER POSITIONED THROUGH ONE END OF SAID HOUSING AND PROVIDING FOR DIRECTING A HOT FLUE GAS STREAM INTO THE INTERIOR THEREOF, A PAIR OF SPACED BLOWERS MOUNTED WITHIN SAID HOUSING AT THE END OPPOSING SAID BURNER, GAS INLET MEANS TO EACH OF SAID BLOWERS FROM THE INTERIOR OF SAID HOUSING, DUCT MEANS FROM THE DISCHARGE OF ONE OF SAID BLOWERS EXTENDING DOWNWARDLY TO A HOT GAS DISTRIBUTING CHAMBER, SAID CHAMBER POSITIONED ABOVE AND CO-EXTENSIVE WITH SAID HORIZONTAL OPEN PASSAGEWAY AND HAVING A PLURALTIY OF OUTLET OPENINGS SPACED ALONG ITS LOWER FACE, A GAS COLLECTING CHAMBER HAVING A PLURALITY OF INLET OPENINGS SPACED ALONG THE UPPER FACE THEREOF AND POSITIONED DIRECTLY BELOW SAID PASSAGEWAY AND SAID BELT PORTION OF SAID CONVEYOR MEANS, A RETURN GAS DUCT EXTENDING ALONG THE INTERIOR SIDE PORTION OF SAID HOUSING FROM SAID GAS COLLECTING CHAMBER TO A GAS OUTLET DISCHARGED AT THE INTERIOR END PORTION OF SAID HOUSING AT A POSTION ADJACENT SAID BURNER WHEREBY A RECIRCULATED GAS STREAM MIXES WITH SAID HOT FLUE GAS STREAM FROM SAID BURNER, EXHAUST GAS DUCT MEANS POSITIONED WITHIN SAID HOUSING AND EXTENDING FROM THE OUTLET OF THE OTHER BLOWER OF SAID PAIR TO AN OXIDIZED GAS OUTLET SECTION, A PERMEABLE LAYER OF AN OXIDATION CATALYST POSITIONED ACROSS THE END PORTION OF SAID EXHAUST GAS DUCT AT THE INLET TO SAID OXIDIZED GAS OUTLET SECTION, AND A TREATED GAS OUTLET FROM THE LATTER SECTON AND FROM SAID HOUSING. 